1. Field:
This invention relates to color printing, and particularly to the process known in the printing arts as color separation in which an original color transparency is used to generate individual halftone separation films (one separation film for each primary color component--yellow, magenta and cyan--and one separation film for black). The halftone color separation films are then used for making printing plates.
2. State of the Art:
Full color printing successively applying primary colored inks --yellow, magenta and cyan--to a sheet from separate halftone printing plates, one plate for each primary color component and one plate for black, in a manner to extend the tone scale and enhance shadow detail. Printed pictures typically start as continuous tone copy, such as photographs, paintings or the like. These must be converted into a dot pattern before they can be printed. The dots vary in size so that the viewer sees them as various tones or shades of the color being printed. If the dots on a printed picture are relatively small, occupying 25% or less of the space in which they are printed, they represent highlight areas. If they are heavy, occupying about 70% or more of the space, they represent shadow areas. In the midtone areas the printed dots occupy 25% to 70% of the space. When a continuous tone copy has been converted into a dot pattern it is called a halftone.
To print a color picture from a color transparency, separate halftone color plates must first be made. This involves the above-mentioned color separation procedure in an electronic color scanner or a digital imaging device. In the scanning process, the original color transparency is mounted on a rotatable transparent analyze drum. Light is directed through the transparency to color sensors which analyze the individual primary color components (yellow, magenta and cyan) and transmit this information electronically to a laser exposure which exposes a halftone dot pattern or a piece of unexposed film mounted on an expose drum. This process produces individual halftone films, one of each of the yellow, magenta and cyan primary colors in the original transparency. A fourth halftone film is also prepared, representing the black component in the original, and a corresponding fourth plate is made from it.
Some original color tranparencies may require an increase or a decrease in one or more color components to achieve a desired color balance, or some special effect. For example, the color densities of the yellow and magenta components may be adjusted for natural flesh tones. This is accomplished by varying the color density (dot size) of the individual color plates involved. For example, if a face is too red, the scanner can be adjusted to reduce the color density of the magenta halftone film, that is, make it with smaller dots. In conventional practice, this is a matter of guesswork, and if it turns out to be wrong, it has to be re-done. Another complication is the probability that changing one color halftone film will require another to be changed. This practice of completely re-making one or more individual color halftone separation films and their corresponding plates is time consuming and costly.